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Equity Compensation Plans - 1996 Employee Stock Purchase Plan (Details Textual) - USD ($) $ / shares in Units, $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Value of stock purchases made pursuant to employee stock purchase plan | $ 2,523 | $ 2,227 | $ 2,311
1996 Employee Stock Purchase Plan |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Number of shares authorized for issue to employees under the employee stock purchase plan |  |  | 1,700,000
Purchase Price of shares as a percentage of market value |  |  | 85.00%
Number of shares purchased under employee stock purchase plan | 68,552 | 54,949 | 68,902
Value of stock purchases made pursuant to employee stock purchase plan | $ 2,500 | $ 2,300 | $ 2,400
Remaining number of shares eligible for purchase under employee stock purchase plan | 153,708 |  |
1996 Employee Stock Purchase Plan | Minimum |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Purchase price of shares under the plan | $ 32.20 | $ 30.76 | $ 27.00
1996 Employee Stock Purchase Plan | Maximum |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |
Purchase price of shares under the plan | $ 42.35 | $ 48.99 | $ 39.68

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